First of all, there is ample evidence that in 1935, when Social Security launched, the actuaries of the Roosevelt administration knew full well that life expectancy at age 65 would gradually extend. So Mr. Ehrlich is simply wrong on his history.

As to policy, here are the facts: Americans who reach age 65 today have a life expectancy of about 19 more years, to age 84. People aged 65 when Social Security started had a life expectancy then of about 14 more years, to age 79. So we've had a modest improvement of five years. But does this five-year average improvement in life expectancy justify moving back the retirement age by five years?

Not if you look closer.

We know that the top half of U.S. wage earners today have a life expectancy at age 65 that is at least five years longer, to age 87, than the bottom half of earners, who at age 65 can expect to live on average to 82 — very little more than the average American of the 1930s. It turns out life's pretty hard on you when you don't make a lot, and the less you make, the harder it is.

So for those people who need Social Security the most, the impact on them of following Mr. Ehrlich's suggestion would be disproportionately awful — it would represent a cut in the total years' benefits received from the program from the current 17 years to 12. That's a reduction of almost 30 percent.

If the size of the U.S. economy per American citizen — per capita GDP — had shrunk by 30 percent since 1935, you might be able to argue that such a cut was justified, assuming public indifference to the plight of old folks who've gone without much. But the size of U.S. GDP per capita has actually increased from 1935's $586 per person to $53,548 in 2013.

Why would a serious student of policy, as Mr. Ehrlich presents himself, suggest that an economy that has grown per capita by a factor of 90 can no longer afford to give retirees a Social Security benefit that we could afford even in the depths of the Depression in 1935?

Either Mr. Ehrlich is actually unacquainted with the facts, or he has decided that defending the owners of the American economy from the taxes required to provide this support to U.S. citizens in their old age is a higher priority for him.

To put it bluntly, what side of this issue you choose to come down on really depends on whom you are for: the relative few people who own the U.S. economy, or the people who do all the work here.

In my father's time (he was a member of "the Greatest Generation"), these groups were much the same. But owing to the past 30 years of wealth concentration, declining middle-class wages and standards of living, that is no longer true.

There is nothing in demographics or economics to support raising the retirement age, and Mr. Ehrlich and his political party should smarten up and stop their repeated attempts to lower the value of this popular and necessary benefit.